Sodium specific glass electrodes

ABSTRACT

The present invention concerns sodium silicate and sodium aluminosilicate glass compositions which contain specified proportions of tantalum oxide and glass electrodes made therefrom, which are particularly sensitive to sodium ions in aqueous solutions containing sodium ions and other monovalent cations. These glasses represent an improvement over sodium aluminosilicate glass elctrodes of the type disclosed in U.S. Pat. No. 2,829,090.

The problem of determining sodium ions in aqueous solutions containingsodium ions and other monovalent cations is well known in the analysisof specimens of industrial and medical interest, and there has been agreat deal of research work directed to the design and manufacture ofelectrodes for carrying out such determinations. In the past various ionselective glass electrodes have been reported and the following patentsand publications are believed to be representative of thestate-of-the-art in this field. "Glasses for Measurements of pH" byGeorge A. Perley, Analytical Chemistry, 21, No. 3 (1949) pp. 394-401;"Glass Electrode for Measuring Sodium Ion" by George Eisenman, Donald O.Rudin, and James U. Casby, Science, 126, (1957) pp. 831-834; "TheBehaviour of the Glass Electrode in Connection with its ChemicalComposition" by B. Lengyel and E. Blum, Faraday Society Transactions,30, (1934) pp. 461-471; "Composition of pH-Responsive Glasses" by GeorgeA. Perley, Analytical Chemistry, 21, No. 3 (1949) pp. 391-394; U.S. PatNo. 3,041,252; U.S. Pat. No. 3,450,604; U.S. Pat No. 3,459,641; U.S.Pat. No. 2,444,845; U.S. Pat. No. 3,713,992; and U.S. Pat. No.2,497,235.

One of the more practical glass electrodes to date is disclosed in U.S.Pat. No. 2,829,090 which provides a glass electrode for selectivelymeasuring sodium ions in ionic mixtures including potassium and hydrogenions wherein the electrode comprises a soda alumina silicate glasshaving the ratio of the mole % of alumina to soda of at least 1 to 1.The particularly preferred composition of this patent contains 11 mole %soda, 18 mole % alumina, and 71 mole % silica.

The sensitivity of the electrode glass in measuring sodium ions withrespect to other monovalent cations is described in terms of theequation used in U.S. Pat. No. 2,829,090 which is set forth below.##EQU1## wherein:

E = measured E.M.F.

E° = standard potential

R = ideal gas equation constant

T = temperature (absolute)

F = Faraday constant

(A⁺) and (B⁺) = activity of the ionic species A⁺ and B⁺

n_(AB) and k_(AB) = empirical constants for a given glass compositionand ionic pair A⁺ and B⁺.

This equation is widely used in describing sodium specific electrodesand will be used in presenting data and properties for the compositionsof the invention.

The present invention represents a specific advance over the sodiumaluminosilicate glasses of the prior art by providing sodiumaluminosilicate and sodium silicate glass compositions containingspecified critical proportions of tantalum oxide to unexpectedly extendthe range compositions which can be effectively used in measuring sodiumions in the presence of other monovalent cations such as K⁺ and NH₄ ⁺ aswell as enhance the sensitivity to such measurements. The affect of H⁺can be minimized by carrying out such measurements at pH above 7.

For practical utility, in a sodium ion selective electrode, the glassesof U.S. Pat. No. 2,829,090 are limited to compositions wherein the Al₂O₃ to Na₂ O ratio (mole % basis) is at least 1.0. The glasses of thesubject case have Al₂ O₃ to Na₂ O ratios of less than 1.0. Thesemodifications have provided glasses with generally lower melting andworking temperatures than those in the prior art sodiumoxide-alumina-silica ternary system and, most importantly, when madeinto electrodes the sodium ion to potassium ion selectivity orsensitivity is increased.

In attaining the objectives of the present invention, one featureresides in a glass composition for use in a sodium selective electrodeconsisting essentially of:

    ______________________________________                                        Component        Mole %                                                       ______________________________________                                        Na.sub.2 O       5-25%                                                        Al.sub.2 O.sub.3 0-20%                                                        Ta.sub.2 O.sub.5 1-10%                                                        SiO.sub.2        60-82%                                                              Ta.sub.2 O.sub.5 + Al.sub.2 O.sub.3                                    wherein            ≧                                                                            0.2                                                         Na.sub.2 O                                                             ______________________________________                                    

For efficiency and economy, the glass compositions consist essentiallyof:

    ______________________________________                                        Component        Mole %                                                       ______________________________________                                        Na.sub.2 O       10-20%                                                       Al.sub.2 O.sub.3  2-15%                                                       Ta.sub.2 O.sub.5 2-8%                                                         SiO.sub.2        65-80%                                                                Ta.sub.2 O.sub.5 + Al.sub.2 O.sub.3                                  wherein 1≧    ≧                                                                            0.2                                                         Na.sub.2 O                                                           ______________________________________                                    

In addition to the specifically named components, the glass compositioncan contain minor proportions (e.g. up to about 3 mole%) of other glassforming or modifying components or refining agents such as calciumoxide, barium oxide, rare earth oxides, or batch material impurities solong as these ingredients do not detrimentally affect the selectivity ofthe glass toward the sodium ion.

It is recognized that the use of tantalum oxide is incidentallydisclosed in lithia silicate glasses in U.S. Pat. Nos. 2,444,845;2,497,235; and 3,713,992, but these lithia silicate compositions arequite different from the sodium containing glasses of invention.

The techniques for melting and refining the glass compositions ofinvention are well established in the art and no unusual techniques arerequired. Suffice it to say that conventional high purity (e.g. reagentgrade) batch material are usually melted in refractory vessels such asplatinum to minimize the concentration of undesirable impurities.Electric or gas fired furnaces in an air atmosphere at temperatures of1600°-1700° C are quite satisfactory.

The glass compositions described above can be used as the glass sensingmembrane of any practical shape and design in the so-called "glasselectrode". For instance, glass electrodes having a bulbous sensingmembrane as disclosed in U.S. Pat. Nos. 2,809,090 or 3,649,505 can beformed; glass electrodes having disc shaped sensing membrane asdisclosed in U.S. Pat. 3,806,440 can be formed; or a wide variety ofother shapes and sizes of sensing membranes such as disclosed in U.S.Pat. No. 2,756,203 or the text "Electrometric pH Determinations" byRoger G. Bates (John Wiley & Sons, Inc. New York) can be employed. Theterm "sensing membrane" is used herein consistent with its usage inpotentiometric electrode technology, and is intended to embrace a flat,bulbous or other curved electrode tip, which provides a pair of surfacesbetween which change transfer is affected.

Similarly, the technique for calibration and measurement of sodium ionsusing the electrodes of the present invention are well established inthe art such as described in the patents and publications mentionedabove.

Other objectives and features of the present invention will become moreapparent from the following description drawing which is a schematicrepresentative of equipment for use in practicing the present invention.

Referring now to FIG. 1 conventional equipment is illustrated formeasuring cation concentration or activity and employing one embodimentof an otherwise conventional glass electrode 10 with a sensing membrane11 made of a glass composition of invention. The glass electrode 10 iselectrically connected to a standard half-cell electrode 12, such assaturated KCl calomel, or silver-silver chloride electrode by means of ahigh impedance, potentiometer such as a conventional laboratory "pHmeter" as described in the Bates text described above. Electrodes 11 and12 are shown as being immersed in a vessel of aqueous test specimen.

The potentiometer can be calibrated with known aqueous solutionscontaining known concentrations of sodium ion as is well known in theart, and then the Na⁺ concentration of unknown solutions can bedetermined directly by subjecting the calibrated glass electrode andreference electrode to the unknown solutions according to the usualprocedure. For example, the equation described above can be simplified(as in U.S. Pat. No. 2,829,090) for mixtures having a wide range of pH(e.g. approximately 3 to 11).

    E = E° + RT/F ln [(Na.sup.+) + K.sub.NaK (K.sup.+)]

wherein:

E = measured E.M.F.

E° = standard potential

R = ideal gas equation constant

T = absolute temperature

F = Faraday constant

(Na⁺) and (K⁺) = activity of the ionic species Na⁺ and K⁺, respectively

K_(NaK) = empirical constant for a given glass composition and ionicpair Na⁺ and K⁺.

In the examples that follow, all percentages are mole percentages, andall temperatures are in ° C unless otherwise stated.

EXAMPLE 1

A glass composition consisting essentially of:

    ______________________________________                                        Component        Mole %                                                       ______________________________________                                        Na.sub.2 O       13.0                                                         Al.sub.2 O.sub.3  5.2                                                         SiO.sub.2        75.8                                                         Ta.sub.2 O.sub.5  6.0                                                                Ta.sub.2 O.sub.5 + Al.sub.2 O.sub.3                                    wherein            =     0.86                                                        Na.sub.2 O                                                             ______________________________________                                    

is prepared by melting appropriate proportions of reagent grade batchmaterials silica, alumina, sodium carbonate, and tantalum pentoxide in aplatinum vessel, in an electric furnace under an air atmosphere at atemperature of 1620° to 1670° C for 21 hours. The molten glass isperiodically manually stirred during melting to assist melting. Theresulting molten glass has a faint yellow-amber coloration.

After melting, the molten glass is poured as a slab into a steel moldand annealed at 650° C for 1 hour. The resulting glass has the followingcharacteristics:

Fiber Softening Point -- 998° C

Annealing Point -- 776° C

Strain Point -- 711° C

Volume Resistivity -- 1.2 × 10¹⁰ ohm-cm.

A glass electrode is prepared by remelting the glass prepared above andcollecting a gob of the molten glass on a chemically durable,borosilicate glass stem. The stem is in the form of a thin walled tubehaving an outside diameter of about 1/3 inch. The molten glass gob isthen blown to form a bulbous sensing membrane on the borosilicate glassstem.

The resulting glass electrode is filled with a 0.1N sodium chlorideelectrolyte solution and a Ag/AgCl electrode is immersed in it. Theassembled electrode is connected to a conventional high impedancepotentiometer (i.e. an Orion Model 801 pH meter) along with a standardsilver/silver chloride reference electrode. The ion-selective andreference electrode are immersed in sodium chloride solutions which varyin concentration from 1 × 10.sup.⁻¹ to 1 × 10.sup.⁻⁵ Molar. The pH isadjusted to 12 for each solution with calcium hydroxide. Theelectromotive force, EMF, or potential is measured for the solutions andrecorded in millivolts. A plot of the logarithm of the sodium ionconcentration versus the electrode potential provides a straight linewith a slope of 58 mv which indicates Nerstian response. This plot isused as a calibration graph for determining the sodium ion concentrationof solutions containing unknown amounts of sodium.

The relative sensitivity of this glass electrode for sodium ions incomparison to potassium and ammonium ions is determined frommeasurements of electrode potentials produced by solutions of sodiumchloride, potassium chloride, and ammonium chloride. In this procedure,0.10 Normal solutions of NaCl, KCl and NH₄ Cl, [the latter adjusted topH 7.0 with tris (hydroxymethyl) aminomethane (TRIS)] give EMF responsesof 84.6, -97.5 and -141.3 millivolts respectively. This data is used tocalculate relative cation sensitivities of the electrode following aprocedure described in U.S. Pat. No. 2,829,090.

By this technique the glass electrode is found to be about 1400 timesmore sensitive to Na⁺ ions than K⁺ ions [i.e. K_(KNa) = 1400] and 8,000times more sensitive to Na⁺ ions than NH₄ + ions [i.e. K_(NH4Na) =8000]. In a similar manner the electrode potential produced by a TRISsolution at pH 8.2 is found to be -161.0 mV. Using the same calculationprocedure, the electrode is approximately 1100 times more sensitive tohydrogen ions than sodium ions [i.e. K_(HNa) = 1.1 × 10.sup.⁻³ ] butthis is not a problem in sodium ion determinations at elevated pH.

CONTROL

The importance of Ta₂ O₅ in glasses of invention is demonstratedpreparing a glass electrode like that of Example 1 using a sensingmembrane having the same mole % of Na₂ O and Al₂ O₃ but no Ta₂ O₅. Thusthe glass composition is:

Na₂ O -- 13.0%

Al₂ O₃ -- 5.2%

SiO₂ -- 81.8%

The glass is used to prepare an electrode as in Example 1 and electrodepotential measurements are made of 0.10 Normal solutions of NaCl, KCland NH₄ Cl. Using the calculation procedures of Example 1 K_(KNa) = 3and K_(NH4Na) = 90.

EXAMPLE 2

The tantalum oxide containing glass compositions of the presentinvention generally have lower melting and working temperatures then thesodium aluminosilicate glass compositions of the prior art forcompositions having similar ion selectivity ratios. Comparison is madeto a sodium ion selective glass having the preferred composition of U.S.Pat. No. 2,829,090. All selectivity measurements are made as describedin Example 1.

    ______________________________________                                                    Glass A Glass B   Prior Art                                       ______________________________________                                        Na.sub.2 O     18        13.0     11                                          Al.sub.2 O.sub.3                                                                             3.6       5.2      18                                          SiO.sub.2      70.4      77.8     71                                          Ta.sub.2 O.sub.5                                                                             8.0       4.0                                                  Selectivity                                                                   K.sub.KNa      480       700       500                                        Selectivity                                                                   K.sub.NH.sbsb.4Na                                                                            2000      6000     3000                                        Fiber Softening Pt.                                                                          921° C                                                                           940° C                                                                            1085° C                            Annealing Pt.  668°                                                                             690°                                                                             790°                                Strain Pt.     610°                                                                             630°                                                                             726°                                ______________________________________                                    

EXAMPLE 3

To further demonstrate the principles of the present invention, severalglass compositions containing various proportions of Ta₂ O₅ are preparedand evaluated by the procedures of Example 1. The results are set forthbelow in Table I.

Table II presents the properties of several prior art sodiumaluminosilicate glasses for the purpose of comparison.

                                      Table I                                     __________________________________________________________________________    Glass   A    B    C   D   E   F    G                                          __________________________________________________________________________    Na.sub.2 O                                                                            23.0 18.0 20.0                                                                              13.0                                                                              15.0                                                                              18.0 18                                         Al.sub.2 O.sub.3                                                                      4.6  3.6  10.0                                                                              10.0                                                                              12.0                                                                              15.0 3.6                                        SiO.sub.2                                                                             70.4 76.4 68.0                                                                              74.0                                                                              70.0                                                                              64.0 74.4                                       Ta.sub.2 O.sub.5                                                                      2.0  2.0  2.0 3.0 3.0 3.0  4.0                                        Ta.sub.2 O.sub.5 /Na.sub.2 O                                                          0.087                                                                              0.11 0.10                                                                              0.23                                                                              0.20                                                                              0.167                                                                              0.22                                       Al.sub.2 O.sub.3 /Na.sub.2 O                                                          0.20 0.2  0.5 0.77                                                                              0.80                                                                              0.83 0.20                                       Ta.sub.2 O.sub.5 +Al.sub.2 O.sub.3                                                    0.287                                                                              0.311                                                                              0.6 1   1   1    0.42                                        Na.sub.2 O                                                                   Selectivity                                                                   Factor                                                                        K.sub.KNa                                                                             4.2  10   200 400 400 400  33                                         K.sub.NH.sbsb.4Na                                                                     110  400  1000                                                                              2000                                                                              2000                                                                              2000 900                                        K.sub.HNa                                                                             1×10.sup.-.sup.3                                                             2×10.sup.-.sup.3                                                                              1×10.sup.-.sup.3                     Fiber Softening                                                               Point             836° C                                               Annealing Point   612°                                                 Strain Point      563°                                                 Glass   H    I    J   K   L   M    N   O                                      __________________________________________________________________________    Na.sub.2 O                                                                            13.0 13.0 20.0                                                                              13.0                                                                              10.0                                                                              18.0 20.0                                                                              13.0                                   Al.sub.2 O.sub.3                                                                      2.6  5.2  10.0                                                                              5.2 2.0 3.6  4.0                                        SiO.sub.2                                                                             80.4 77.8 65.0                                                                              75.8                                                                              82.0                                                                              70.4 68.0                                                                              81.8                                   Ta.sub.2 O.sub.5                                                                      4.0  4.0  5.0 6.0 6.0 8.0  8.0 5.2                                    Ta.sub.2 O.sub.5 /Na.sub.2 O                                                          0.31 0.31 0.25                                                                              0.46                                                                              0.60                                                                              0.44 0.40                                                                              0.4                                    Al.sub.2 O.sub.3 /Na.sub.2 O                                                          0.20 0.40 0.50                                                                              0.40                                                                              0.20                                                                              0.20 0.20                                       Ta.sub.2 O.sub.5 +Al.sub.2 O.sub.3                                                    0.51 0.71 0.75                                                                              0.86                                                                              0.80                                                                              0.64 0.60                                                                              0.4                                     Na.sub.2 O                                                                   Selectivity                                                                   Factor                                                                        K.sub.KNa                                                                             300  700  400 1300                                                                              1100                                                                              480  1000                                                                              220                                    K.sub.NH.sbsb.4Na                                                                     1000 6000 2000                                                                              8000                                                                              3000                                                                              2000 1000                                                                              1000                                   K.sub.HNa                                                                             1×10.sup.-.sup.3                                                             2×10.sup.-.sup.3                                                                 2×10.sup.-.sup.3                                                            2×10.sup.-.sup.3                              Fiber Softening                                                               Point   889° C                                                                      940° C                                                                      910° C                                                                     998° C                                                                     997° C                                                                     921° C                                                                      896° C                                                                     887° C                          Annealing Point                                                                       649°                                                                        690°                                                                        702°                                                                       776°                                                                       798°                                                                       668°                                                                        548°                                                                       714°                            Strain Point                                                                          589°                                                                        632°                                                                        655°                                                                       711°                                                                        740°                                                                      610°                                                                        491°                                                                       664°                            __________________________________________________________________________

                  Table II                                                        ______________________________________                                                P     Q      R       S     T                                          ______________________________________                                        Na.sub.2 O                                                                              20.0    23     18    13.0  11                                       Al.sub.2 O.sub.3                                                                        10.0    4.6    3.6   5.2   18                                       SiO.sub.2 70.0    72.4   78.4  81.8  71                                       Al.sub.2 O.sub.3                                                              Na.sub.2 O                                                                              0.5     0.2    0.2   0.4   1.64                                     K.sub.KNa 30      0.6    1.5   3     500                                      K.sub. NH.sbsb.4Na                                                                      300     2.8    33    90    3000                                     K.sub.HNa                1×10.sup.-.sup.3                                                                    1×10.sup.-.sup.3                   Fiber Softening                                                                         887° C        791° C                                  Point                                                                         Annealing 714°          693°                                    Point                                                                         Strain    664°          643°                                    Point                                                                         ______________________________________                                    

From the foregoing comparative data, it is readily apparent that thepresent invention provides a new family of sodium specific glasscompositions which have improved selectivity as a result of theinclusion of specified proportions of tantalum oxide. For convenience indisclosure, all of the patents and publications mentioned herein areincorporated by reference.

Having thus described the invention, what is claimed is:
 1. In anelectrode having a glass sensing membrane the improvement wherein saidsensing membrane is selectively responsive to sodium ions in an ionicmixture containing sodium and other monovalent cations and has acomposition consisting essentially of:

             Component   Mole %                                                   ______________________________________                                               Na.sub.2 O     5-25%                                                          Al.sub.2 O.sub.3                                                                             0-20%                                                          Ta.sub.2 O.sub.5                                                                             1-10%                                                          SiO.sub.2     60-82%                                                              Ta.sub.2 O.sub.5 + Al.sub.2 O.sub.3                                wherein                ≧                                                                             0.2.                                                       Na.sub.2 O                                                         ______________________________________                                    


2. The glass electrode of claim 1 wherein said sensing membrane has acomposition consisting essentially of:

    ______________________________________                                        Component        Mole %                                                       ______________________________________                                        Na.sub.2 O       10-20%                                                       Al.sub.2 O.sub.3  2-15%                                                       Ta.sub.2 O.sub.5 2-8%                                                         SiO.sub.2        65-80%                                                                Ta.sub.2 O.sub.5  + Al.sub.2 O.sub.3                                 wherein 1≧    ≧                                                                            0.2.                                                        Na.sub.2 O                                                           ______________________________________                                    